The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity for wireless systems by employing uncorrelated propagation paths, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity. MIMO employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas. The desired result is to have a number of uncorrelated antenna ports for receiving as well as transmitting.
For MIMO it is desired to estimate the channel and continuously update this estimation. This updating may be performed by means of continuously transmitting so-called pilot signals in a previously known manner. The estimation of the channel results in a channel matrix. If a number of transmitting antennas Tx transmit signals, constituting a transmitted signal vector, towards a number of receiving antennas Rx, all Tx signals are summated in each one of the Rx antennas, and by means of linear combination, a received signal vector is formed. By multiplying the received signal vector with the inverted channel matrix, the channel is compensated for and the original information is acquired, i.e. if the exact channel matrix is known, it is possible to acquire the exact transmitted signal vector. The channel matrix thus acts as a coupling between the antenna ports of the Tx and Rx antennas, respectively. These matrixes are of the size M×N, where M is the number of inputs (antenna ports) of the Tx antenna and N is the number of outputs (antenna ports) of the Rx antenna. This is previously known for the skilled person in the MIMO system field.
In order for a MIMO system to function efficiently, uncorrelated, or at least essentially uncorrelated, transmitted signals are required. The meaning of the term “uncorrelated signals” in this context is that the radiation patterns are essentially orthogonal. This is made possible for one antenna if that antenna is made for receiving and transmitting in at least two orthogonal polarizations. If more than two orthogonal polarizations are to be utilized for one antenna, it is necessary that it is used in a so-called rich scattering environment having a plurality of independent propagation paths, since it otherwise is not possible to have benefit from more than two orthogonal polarizations. A rich scattering environment is considered to occur when many electromagnetic waves coincide at a single point in space. Therefore, in a rich scattering environment, more than two orthogonal polarizations can be utilized since the plurality of independent propagation paths enables all the degrees of freedom of the antenna to be utilized.
Antennas for MIMO systems may utilize spatial separation, i.e. physical separation, in order to achieve low correlation between the received signals at the antenna ports. This, however, results in big arrays that are unsuitable for e.g. hand-held terminals. One other way to achieve uncorrelated signals is by means of polarization separation, i.e. generally sending and receiving signals with orthogonal polarizations.
It has then been suggested to use three orthogonal dipoles for a MIMO antenna with three ports, but such an antenna is complicated to manufacture and requires a lot of space when used at higher frequencies, such as those used for the MIMO system (about 2 GHz). Up to six ports have been conceived, as disclosed in the published application US 2002/0190908, but the crossed dipole and the accompanying loop element is still a complicated structure that is difficult to accomplish for higher frequencies to a reasonable cost.
The objective problem that is solved by the present invention is to provide an antenna arrangement suitable for a MIMO system, which antenna arrangement is capable of sending and receiving in three essentially uncorrelated polarizations. The antenna arrangement should further be made in a thin structure to a low cost, and still be suitable for higher frequencies, such as those used in the MIMO system.